The present invention pertains to a system for stacking serially delivered sheets and, more particularly, to an apparatus and method for high speed formation and discharge for stack portions of a precounted number of sheets from a continuously forming stack.
Stacking apparatus for paper and paperboard sheets are well known in the art. The manufacture of paper or paperboard products from individual sheets of material typically requires a stacking of the paper or paperboard sheets in at least one step of the manufacturing process. Indeed, many converting processes require that paper or paperboard sheets be stacked more than one time in the overall process. In the manufacture of corrugated paperboard containers, paperboard sheets are typically stacked after leaving the corrugator dry end for eventual feed into the box blank forming apparatus (a flexo-folder-gluer) and the folded knocked down boxes are again restacked after formation in the flexo. In both cases, the number of sheets or sheet-like items in the stack must be accurately counted and separated.
It is common to compress a continuous stream of sheets being delivered for stacking by shingling the stream of sheets upstream of the stacker and feeding the preshingled stream directly into a continuously descending downstacker with the sheets being stopped by engagement with a backstop wall. Typically, the sheets must be slowed for stacking to maintain control and prevent lead edge sheet damage. U.S. Pat. No. 4,966,521 discloses a stacking system in which a continuous stream of sheets which are spaced end-to-end is delivered to a downstacker with sheet slow down and control effected by controlled sequential nipping of the tail ends of the sheets to slow each sheet just before the lead edge contacts the backstop. This patent also discloses a lower stack separating wedge which is driven horizontally into the stack to create a bottom stack portion for separate discharge from the system.
It is also known to utilize vertically reciprocable stop gates or false backstops to provide a staggered offset in a vertically forming stack in response to counting mechanisms to divide the stack into preselected stack portions of a given number of sheets. Examples of such apparatus are shown in U.S. Pat. Nos. 1,366,938; 2,645,476 and 2,839,295.
Folded knocked down boxes from a flexo-folder-gluer are counted and stacked for discharge in one type of device known as a counter ejector. One conventional type of counter ejector is shown in U.S. Pat. No. 3,580,145 in which stacks of folded boxes are individually formed and serially ejected from the apparatus.
U.S. Pat. No. 3,892,168 shows a counter ejector in which a stack of folded boxes is continuously formed on a vertically descending platform in the stacking station. When a precounted number of boxes has been stacked, support fingers move over the top of the stack to intercept the continuous stream of boxes that follows to provide temporary support while the lower precounted stack portion is discharged, after which the platform moves vertically upwardly to the position of the support fingers which are withdrawn from the next stack portion forming thereon.
U.S. Pat. No. 4,134,330 describes a device for feeding folded box blanks serially and individually into a downstacker. When the desired number of blanks in a stack portion has been reached, the first blank of the next following stack portion is deflected laterally from the feed path into the stacker where it is supported on a secondary support mechanism and where the following blanks of the second stack portion are also deposited while the preceding stack portion is being discharged.